1. Field of the Invention
The present invention relates to a substantially lead-free glass flux compositions used for the decoration of ceramic materials and methods for making the same. The present invention also relates to materials for decorating ceramic materials which contain said glass flux compositions and methods for making the same.
2. Description of the Related Art
Generally, a material for decorating a ceramic material is composed of one or more pigments and a glass flux. For example, Japanese Laid-Open Patent Publication No. 6-56558 describes a decorating material for decorating the surface of a ceramic material with noble metal. As a glass flux for use in a decorating material, it is preferable to use a glass flux mainly composed of SiO2 (typically, a glass flux having a SiO2 content of 45% by mass or more) in order to achieve clear development and luster for a decoration (hereinafter also referred to as “painting”) formed by the decoration and to improve the water resistance, chemical resistance such as acid resistance, alkali resistance and detergent resistance required for ceramic materials of the painting.
In so-called on-glaze (overglaze) decoration, a decorating material (paint) including various pigments (coloring components) is applied to a glazed surface that has been previously formed on the surface of a ceramic material (e.g., a glaze layer that is formed on porcelain with a hard glaze), and is then fired at a relatively low temperature of about 750 to 900° C. Accordingly, the glass flux contained in such a decorating material for on-glaze decoration is required to have a lower melting temperature and a lower softening point, which is the index of a melting temperature, than those of a glass flux contained in a decorating material for in-glaze decoration, which is normally fired at 1000 to 1300° C. On the other hand, the softening point of a glass composition generally tends to increase with an increase in its SiO2 content. For this reason, it is preferable that glass flux contained in a decorating material for on-glaze decoration has a low softening point suitable for on-glaze decoration, while maintaining the SiO2 content at a relatively high level.
In order to satisfy the above-described requirements, glass compositions (lead-containing glasses), which contain lead (Pb) as a component for lowering the softening point, have conventionally been used as the glass flux for a decorating material for on-glaze decoration. For example, Japanese Laid-Open Patent Publication No. 7-267677 discloses a lead-containing glass flux for use in a decorating material. Presently, lead-free glass compositions, which contain components other than lead that have the function of lowering a softening point, have begun to be used as the glass flux for a decorating material for on-glaze decoration. For example, Japanese Patent No. 2991370 (Japanese Laid-Open Patent Publication No. 8-259258) describes decorating materials for on-glaze decoration that contain a lead-free glass composition which is based within the Li2O—Al2O3—SiO2 system. For example, the lead-free glass composition just described contains, in place of lead, Li2O or other alkali metal oxides as components for promoting melting (i.e., components for lowering the softening point). Japanese Laid-Open Patent Publication No. 10-167756 discloses a lead-free decorating material for on-glaze decoration that contains niobium oxide or tantalum oxide for creating a quality similar to that of a lead-containing decorating material for on-glaze decoration.
However, sufficient consideration has not been given to the conventional lead-free glass fluxes for use in a decorating material, with respect to the thermal expansion. That is, if the thermal expansion rate (the coefficient of thermal expansion) of the glass flux in a decorating material is too much higher than those of the body and glazed surface of a ceramic material, the difference in their thermal expansion may cause such problems as the peeling of a painting and the excessive crazing, when firing the decorating material applied on the glazed surface of the ceramic material at a predetermined temperature. For this reason, it is preferable that the coefficient of thermal expansion of the glass flux is approximately that of the body and glaze of a ceramic material.
However, the conventional lead-free glass compositions used as the glass flux for a decorating material for on-glaze (overglaze) decoration (e.g., a lead-free glass composition with a high SiO2 content that contains an alkali metal oxide as a component for promoting melting) have such an undesirable tendency in that the coefficient of thermal expansion increases with a decreasing softening point (melting temperature).